This invention relates to a plating bath for electrodeposition of aluminum and a process for the same.
U.S. Pat. No. 2,446,331, No. 2,446,349 and No. 2,446,350 disclose a process for electrodeposition of aluminum by use of a nonaqueous electrolyte, which is a process carried out by using a plating bath comprising a mixture (a eutectic mixture) of an aluminum halide and a N-alkyl pyridinium halide. However, it is stated in the above patents that a good aluminum coating can be obtained when the aluminum halide comprises aluminum chloride and the N-substituent for the N-alkyl pyridinium halide comprises an ethyl group, and there is no disclosure as to the examples where the N-substituent for N-alkyl pyridinium halide comprises other alkyl groups.
The N-ethyl pyridinium halide used when a plating bath is prepared is not commercially available since there is no demand therefor even as a reagent. For this reason, it must be synthesized when used for preparing a plating bath. Synthesis therefor can be achieved by reacting an ethyl halide with pyridine. However, the ethyl halide, which is gaseous, has a disadvantage that it is in danger of explosion during its synthesis.
Also, in carrying out the electrodeposition of aluminum in an industrial scale, it has been necessary to carry out plating under high current density in order to make high the plating speed, and also to control appearance to be uniform even with change in the current density. However, plating under high current density with use of a plating bath comprising the mixture of aluminum chloride and N-ethyl pyridinium halide, the appearance may suffer color-changed and constant appearance may be assured only within a very narrow range of the current density. For example, in the instance of the plating bath disclosed in U.S. Pat. No. 2,446,331, the current density at which a coating having appearance of aluminum can be obtained is 0.5 A/dm.sup.2, and the density lower than that may result in a cloudy appearance. On the other hand, the current density higher than that may result in a grayish appearance, and the density excessively higher that that may result in generation of black spots. Also, in the instance of the plating bath disclosed in U.S. Pat. No. 2,446,349, an optimum current density is 0.5 to 1.0 A/dm.sup.2, and the density higher than 1.0 A/dm.sup.2 may result in generation of blown colored streaks. Accordingly, these plating baths can be suited for batch plating for the parts (for example, screw thread, rivets, frames, etc, for aircrafts), but, in the continuous plating where the plating is carried out while a metallic band is moved at constant speedd, the plating can be achieved taking an overly long time to seriously lower the productivity. It is also difficult to make uniform the appearance as a whole of the metallic band.